The present invention relates in general to a device for detecting the location of a machine tool, and more particularly, to a tool setter having one or more machine tool detecting probes which is detachably mounted to a rotatable workpiece holder in precision registration therewith in order to repeatably position the machine tool detecting probes at a predetermined fixed reference location relative to the workpiece holder upon each mounting of the tool setter to the workpiece holder.
In order to precision machine a workpiece in a lathe or other such machine, the operator must know the precise location of the machine tool at initiation of the machining operation. Once the location of the machine tool is known, the path of the machine tool can be accurately controlled during the machining operation to generate the completed workpiece.
The setting of the machine tool on a lathe was traditionally a manual operation in which the position of the machine tool in a tool holder was measured and adjusted so that its cutting tip was in a defined position. In the case of the height of the tip above the lathe bed, this was achieved by packing shims between the machine tool and tool holder until the tip height coincided with the center line of the lathe (the axis of rotation of its workpiece holder). The direction of this height adjustment may be referenced to as the Y direction of the lathe, based on the convention that the axis of rotation of the workpiece holder defines the Z axis and the tool holder is mounted for sliding movement in at least the X and Z directions.
On a modern computer numerically controlled lathe, it is known to provide a tool setter having a machine tool detecting probe in a defined position relative to the lathe bed. The probe is provided with a trigger mechanism, such as described in U.S. Pat. No. 4,153,998, which provides an output signal at the instant of contact with the machine tool to cause the lathe's controlling computer to take a reading of the machine tool's position from a measurement scale of the lathe. The difference between the measurement and the known position of the datum surface is used by the computer as an offset value for the machine tool tip during subsequent machining operations. One such tool setter and method of using same is described in U.S. Pat. No. 4,382,215.
In conventional lathes, a tool setter is fixedly mounted to the lathe bed or to a pivotable arm and includes at least one machine tool detecting probe which is located at a known distance from the lathe's center line, i.e., rotational axis of the workpiece holder. One such tool setter is known from U.S. Pat. No. 4,879,817. By moving the machine tool in contact with the probe, the location of the machine tool relative to the center line of the workpiece holder will be known and the path the machine tool will have to take in order to generate the desired workpiece can be accurately calculated.
There are a number of notable disadvantages which result from the permanent mounting of a tool setter on a lathe such as known from U.S. Pat. No. 4,879,817. For example, the machine tool detecting probes on the tool setter are subject to contamination by machine chips and/or coolant which are spewn during the machine cutting operation. Further, the probes cannot be located very close to the workpiece holder center line since they will interfere with the machine cutting operation. Minimizing the distance between the probes and the workpiece holder center line is significant because changes in the temperature of the lathe, even a few decrees, will cause its thermal expansion which will effect the accuracy of the tool setter in direct proportion to the probe's distance from the workpiece holder center line. The resulting inaccuracy in locating the machine tool will adversely effect the accurate dimensioning of the workpiece during the machine cutting operation.